I am a happy user of SailfishOS. Have been ever since I bought the original Jolla in 2013.

One big benefit is the UI. It’s based heavily on swiping, which suits me, and many have complained about the idea until they took the risk and fell in love. It’s also not a huge geeky thing but originally paid for in Nokia’s money.

My current phone is the Fairphone2, which runs a community port quite nicely. The official store works, as does Warehouse, an openrepos.net client for all the hardcore hacker stuff if that’s your thing.

A potential downside is that the community version doesn’t have official support for Android. There is the sfdroid project, and I think it works for some other phones, but not on this one. Not sure how actively developed is, so don’t hold your breath.

Given the thickness of my tin-foil hat, maybe that’s just as fine.. I run Whisperfish to use Signal, which many friends and most family have adopted anyway.

There is some interest to get an official SailfishOS for this phone, but Jolla, the company behind it, is struggling to get new phones out so this is unfortunately not a priority for them.

CopperheadOS is technically still Android, but in my opinion is sufficiently different enough from stock AOSP. The developers have completely stripped out Google Play Services, and added a bunch of security mitigations. Apps must be installed from the F-Droid “store”.

CopperheadOS has a strong security focus. I would wager they are the most secure Android ROM.

Originally Copperhead wanted to base their work on Cyanogenmod in order to get broad device support “for free”, but they found they were spending too much effort undoing Cyanogen changes. Currently their work is based directly on vanilla AOSP. Unfortunately this means they only support a handful of devices right now: Nexus 5X, Nexus 6P, Pixel, and Pixel XL. On the bright side, each of those phones gets prompt security updates (being directly supported by the AOSP project).

Copperhead is chock full of security mitigations. Some of their changes have even been merged into upstream AOSP (but Copperhead users got them first). They are using a hardened C library, stronger sandboxing, ASLR in more places, etc.

Notably, Copperhead has completely removed the Google Play Services stack. If you want to install apps, they either need to be sideloaded or installed via F-Droid. Also notably, Copperhead does not provide you with root access to your own phone, since that is a security vulnerability.

Both LineageOS and CopperheadOS get high marks for privacy. LineageOS has better customizability and phone support, whereas CopperheadOS has better security.